Globally (outside of the class), you have to define the static variable:
int HotDogStand::totalSold = 0;
Change
void HotDogStand::justSold()
{
hotDogsSold++;
totalSold++; // increment here
return;
}
And
int HotDogStand::getTotalSold()
{
return totalSold; // just return value
}